Sigismund the Old and Katarzyna Telniczanka. Katarzyna was Sigismund's mistress, he had 3 kids with her. He never married her, because she was not even noblewoman-likely she came from burgher family from Moravia. Thus such marriage would be scandalous, but at the time Sigismund started his affair with Telniczanka (around 1498) his 3 older brothers were still alive, so he had little prospect to get throne at the time, and Sigismund was devout Christian, so perhaps he is afraid, that he and Katarzyna would go to Hell for living in sin, thus he secretly marries her to save their souls?